Conventionally, as an optical signal receiver, optical signal receiving equipment and an optical signal transmission system that transmit multichannel picture signals each of which was amplitude-modulated or quadrature-amplitude-modulated and that have been frequency-division-multiplexed, there are known an optical signal receiver, optical signal receiving equipment, and an optical signal transmission system each of which uses an FM batch conversion method of frequency-modulating collectively picture signals that have been frequency-division-multiplexed.
The optical signal transmitter and the optical signal transmission system that use this FM batch conversion method has been adopted in International Standards ITU-T J. 185 “Transmission equipment for transferring multi-channel television signals over optical access networks by FM conversion” (see Non-patent document 1).
FIG. 1 shows the configuration of the optical signal receiver and the optical signal transmission system that use the conventional FM batch conversion method. FIG. 2A, FIG. 2B, and FIG. 2C show signal spectra in positions of A, B, and C in FIG. 1. The optical signal transmission system shown in FIG. 1 comprises: an optical signal transmitter 80 having an FM batch conversion circuit 81, a light source 82, and an optical amplifier circuit 83; an optical transmission path 85; an optical signal receiver 90 having a photoelectric conversion circuit 91 and an FM demodulator circuit 92; a set top box 93; and a television receiver 94. FIG. 2A, FIG. 2B, and FIG. 2C show spectra of A, B, C in FIG. 1, respectively. This correspondence is the same for spectra of A, B, and C in subsequent figures.
In FIG. 1, in the optical signal transmitter 80, the picture signals that have been frequency-division-multiplexed as shown in FIG. 2A are converted to a single frequency-modulated signal that occupies a wide band, as shown in FIG. 2B, by the FM batch conversion circuit 81. The frequency-modulated signal is allowed to intensity-modulate the light source 82. Further, the optical signal is amplified by the optical amplifier circuit 83 and transmitted in the optical transmission path 85. In the optical signal receiver 90, the optical signal is returned to the electrical signal by photoelectric conversion in the photoelectric conversion circuit 91. This electrical signal is a wide-band frequency-modulated signal, which is frequency-demodulated by the FM demodulator circuit 92 to yield picture signals that have been frequency-division-multiplexed, as shown in FIG. 2C. The picture signals thus demodulated are selected to display an appropriate video channel on the television receiver 94 through the set top box 93.
FIG. 3 shows the configuration of an FM batch conversion circuit using an optical-frequency modulation unit and an optical-frequency local oscillator unit that can be applied to this FM batch conversion method (for example, see Patent document 1, Non-patent document 2, and Non-patent document 3). The FM batch conversion circuit 81 shown in FIG. 3 comprises an optical-frequency modulation unit 101, an optical multiplexer unit 102, a photodiode 103 as an optical detection unit, and an optical-frequency local oscillator unit 104.
Consider the optical-frequency modulation unit 101 frequency-modulating a carrier light source of optical frequency fo at a frequency fs in the FM batch conversion circuit 81. Representing a frequency shift by δf, an optical frequency of the optical signal Ffmld at an output of the optical-frequency modulation unit 101 is given byFfmld=fo+δf·sin(2π·fs·t).  (1)As the carrier light source of the optical-frequency modulation unit 101, a DFB-LD (Distributed Feed-Back Laser Diode, distributed feedback semiconductor laser) is used.
The optical-frequency local oscillator unit 104 makes the oscillation light source oscillate a light of optical frequency fl, which is multiplexed with an optical signal from the optical-frequency modulation unit 101 in the optical multiplexer unit 102. A DFB-LD is used as the oscillation light source of the optical-frequency local oscillator unit 104. Two optical signals multiplexed in the optical multiplexer unit 102 are detected in an optical detection unit 103. The opto-heterodyne is applied as a detection method and a photodiode is used as the detection element. The frequency f of the detected electrical signal is given byf=fo−fl+δf·sin(2π·fs·t).  (2)Here, if the optical frequency of the carrier light source of the optical-frequency modulation unit 101 and that of the oscillation light source of the optical-frequency local oscillator unit 104 are brought close to each other, an electrical signal whose intermediate frequency fi=fo−fl is a few GHz and that is frequency-modulated with a frequency shift δf, as shown in FIG. 2B, can be obtained.
Since generally, when the DFB-LD is modulated by an injected current, its optical frequency varies with the injection current in a width of a few GHz, a value of a few GHz can be obtained as the frequency shift δf. For example, multichannel AM picture signals or QAM picture signals that have been frequency-division-multiplexed in a frequency span ranging from about 90 MHz to about 750 MHz can be converted into a frequency-modulated signal in a band of about 6 GHz whose intermediate frequency fi=fo−fl is about 3 GHz, as shown in FIG. 2A, using the FM batch conversion circuit.
FIG. 4 shows an example of another FM batch conversion circuit that is applied to this FM batch conversion method and that uses two optical-frequency modulation units in a push-pull configuration. The FM batch conversion circuit 81 shown in FIG. 4 comprises a differential distributor 105, an optical-frequency modulation unit 106, an optical-frequency modulation unit 107, an optical multiplexer unit 102, and a photodiode as the optical detection unit 103.
In the FM batch conversion circuit 81, the picture signals that have been frequency-division-multiplexed as shown in FIG. 2A are distributed to two electrical signals whose phases are inverted to each other in the differential distribution unit 105. When frequency modulation is performed on an optical signal using one electrical signal out of the two electrical signals from the differential distribution unit 105 as the modulation input and also using the carrier light source of optical frequency fo1 in the optical-frequency modulation unit 106, the optical frequency Ffmld1 of the optical signal at an output of the optical-frequency modulation unit 106 is given by the following formula in the case of a frequency shift δf/2,Ffmld1=fo1+(δf/2)·sin(2π·fs·t).  (3)Here, in Formula (3), the modulation signal is assumed to be a signal of frequency fs. When frequency modulation is performed using the other electrical signal out of the two electrical signals from the differential distribution unit 105 as the modulation input and also using the carrier light source of optical frequency fo2 in the optical-frequency modulation unit 107, the optical frequency Ffmld2 of the optical signal at the output of the optical-frequency modulation unit 106 is given by the following formula in the case of a frequency shift δf/2,Ffmld2=fo2−(δf/2)·sin(2π·fs·t)  (4)In Formula (4), the modulation signal is assumed to be a signal of frequency fs. As carrier light sources of the optical-frequency modulation units 106 and 107, DFB-LDs (Distributed Feed-Back Laser Diodes, distributed feedback semiconductor lasers) can be used.
The outputs from the optical-frequency modulation units 106 and 107 are multiplexed in the optical multiplexer unit 102, and two optical signals multiplexed in the optical multiplexer unit 102 are heterodyne-detected in the optical detection unit 103. As the optical detection unit, photodiodes functioning as heterodyne detection elements can be used. The electrical signal that was heterodyne-detected in the optical detection unit 103 is an electrical signal whose frequency f equals a difference between values expressed by Formula (3) described above and by Formula (4) described above. That is, the frequency is given byf=fo1−fo2+δf·sin(2π·fs·t).  (5)However, the modulation signal is assumed to be a signal of frequency fs in Formula (5). Here, if the optical frequency of the carrier light source of the optical-frequency modulation unit 106 and that of the oscillation light source of the optical-frequency local oscillator unit 107 are brought close to each other, an electrical signal whose intermediate frequency fi=fo−fl is a few GHz and that is frequency-modulated with a frequency shift δf, as shown in FIG. 2B, can be obtained.
Generally, when the DFB-LD is modulated by an injected current, the optical frequency thereof is varied in a width of a few GHz in accordance with the injected current; therefore, a frequency shift δf of a few GHz can be obtained. For example, multichannel AM picture signals or QAM picture signals that have been frequency-division-multiplexed in a frequency span ranging from about 90 MHz to about 750 MHz can be converted into a frequency-modulated signal in a band of about 6 GHz whose intermediate frequency fi=fo−fl is set to about 3 GHz, as shown in FIG. 2B, by the FM batch conversion circuit.
FIG. 5 shows another FM batch conversion circuit that is applied to this FM batch conversion method and that uses a voltage controlled oscillation element. The FM batch conversion circuit 81 shown in FIG. 5 is equipped with a voltage controlled oscillation unit 111 using a voltage controlled oscillation element.
In the FM batch conversion circuit 81, when the picture signals that have been frequency-division-multiplexed as shown in FIG. 2A are frequency-modulated using a frequency fo as a center frequency in the voltage controlled oscillation unit 111, the frequency fv of an output electrical signal is given by the following formula in the case of a frequency shift δf.fv=fo+δf·sin(2π·fs·t)  (6)Thus, the frequency-modulated signal with an intermediate frequency fi=fo and a frequency shift δf can be obtained. Note that the modulation signal is assumed to be a signal of frequency fs in Formula (6).
For example, multichannel AM picture signals or QAM picture signals that have been frequency-division-multiplexed in a frequency span ranging from about 90 MHz to about 750 MHz can be converted into a frequency-modulated signal in a band of about 6 GHz, as shown in FIG. 2B, by the FM batch conversion circuit 81 with an intermediate frequency fi=fo being set to about 3 GHz.
FIG. 6 shows an example of another FM batch conversion circuit that is applied to this FM batch conversion method and that uses two voltage controlled oscillation elements in a push-pull configuration. The FM batch conversion circuit 81 shown in FIG. 6 comprises the differential distribution unit 105, a voltage controlled oscillation unit 112, a voltage controlled oscillation unit 114, a mixer 115, and a low pass filter 117.
In the FM batch conversion circuit 81, the picture signals that have been frequency-division-multiplexed as shown in FIG. 2A are distributed to two electrical signals whose phases are inverted to each other in the differential distribution unit 105. When, using one electrical signal out of the two electrical signals from the differential distribution unit 105 as a modulation input, frequency modulation that uses a frequency fo as the center frequency is performed in the voltage controlled oscillation unit 112, the frequency fv1 of the output electrical signal is given by the following formula in the case of a frequency shift δf/2,fv1=fo1+(δf/2)·sin(2π·fs·t).  (7)That is, a frequency-modulated signal with an intermediate frequency fi=fo1 and a frequency shift δf/2 is obtained. In Formula (7), the modulation signal is assumed to be a signal of frequency fs. When, using the electrical signal out of the two electrical signals from the differential distribution unit 105 as a modulation input, frequency modulation that uses a frequency fo1 as the center frequency in the voltage controlled oscillation unit 114 is performed, the frequency fv2 of the output electrical signal is given by the following formula in the case of a frequency shift δf/2,fv2=fo2−(δf/2)·sin(2πfs·t)  (8)A frequency-modulated signal with an intermediate frequency fi=fo2 and a frequency shift δf/2 is obtained. In Formula (8), the modulation signal is assumed to be a signal of frequency fs.
The outputs from the voltage controlled oscillation units 112 and 114 are mixed by the mixer 115, and a signal into which the two electrical signals were mixed by the mixer 115 is smoothed by the low pass filter 117. The electrical signal smoothed by the low pass filter 117 that passes an electrical signal of a frequency equal to a difference between the intermediate frequency fo1 and the intermediate frequency fo2 becomes an electrical signal whose frequency equals a difference between values expressed by Formula (7) described above and by Formula (8) described above. That is, the frequency is given byf=fo1−fo2+δf·sin (2π·fs·t).  (9)In Formula (9), the modulation signal is assumed to be a signal of frequency fs. Here, an electrical signal whose intermediate frequency fi=fo1−fo2 is a few GHz and that is frequency modulated with a frequency shift δf, as shown in FIG. 2B, can be obtained.
For example, multichannel AM picture signals or QAM picture signals that have been frequency-division-multiplexed in a frequency span ranging from about 90 MHz to about 750 MHz can be converted into a frequency-modulated signal in a band of about 6 GHz, as shown in FIG. 2B, with an intermediate frequency fi=fo1−fo2 being set to about 3 GHz by the FM batch conversion circuit.
Heretofore, as a technique aiming at reduction of distortion, a pre-distortion circuit is known (for example, see Patent document 2). FIG. 7 shows the configuration of an optical signal transmission system using the conventional FM batch conversion method in which a predistortion circuit is applied to distortion compensation of the FM batch conversion circuit. The optical signal transmission system shown in FIG. 7 comprises: the optical signal transmitter 80 having a predistortion circuit 86, the FM batch conversion circuit 81, the light source 82 as a transmitter circuit, and the optical amplifier circuit 83; the optical transmission path 85; the optical signal receiver 90 having the photoelectric conversion circuit 91 and the FM demodulator circuit 92; the set top box 93; and the television receiver 94. Signal spectra A, B, and C in FIG. 7 become frequency spectra shown in FIG. 2A, FIG. 2B, and FIG. 2C, respectively.
When multichannel AM picture signals or QAM picture signals are inputted into the predistortion circuit 86, the predistortion circuit 86 adds beforehand a distortion inverse to a distortion that the FM batch conversion circuit 81 etc. will generate, and thereby compensates the distortion generated by the subsequent FM batch conversion circuit 81 etc. An output of the predistortion circuit 86 is frequency-modulated by the FM batch conversion circuit 81, converted from the electrical signal to an optical signal by the light source 82, optically amplified by the optical amplifier circuit 83, and subsequently transmitted in the optical transmission path 85. The transmitted optical signal passes through the optical transmission path 85, is converted to an electrical signal by the photoelectric conversion circuit 91 of the optical signal receiver 90, and frequency-demodulated to yield the original AM picture signals or QAM picture signals by the FM demodulator circuit 95.
FIG. 8 shows an example of the configuration of the predistortion circuit. The predistortion circuit 86 shown in FIG. 8 comprises an inphase distribution unit 121, a delay line 122, a distortion generator circuit 123, an amplitude adjusting unit 124, a delay adjusting unit 125, and a differential combining unit 126. The multichannel AM picture signals or QAM picture signals inputted into the inphase distribution unit 121 are split into two sets of signals. One set of split signals is added with a distortion that will generate in the FM batch conversion circuit etc. by the distortion generation circuit 123, and their amplitudes and delays are adjusted by the amplitude adjusting unit 124 and the delay adjusting unit 125, respectively. The other set of split signals is delayed by the delay line 122. The signals outputted from the delay adjusting unit 125 and those outputted from the delay line 122 are combined in the differential combining unit 126. As a result, the signal outputted from the differential combining unit 126 becomes a signal to which a distortion inverse to a distortion that the FM batch conversion circuit etc. will generate is added beforehand.
On the other hand, as a frequency demodulator circuit method, there is a delay line detection method. FIG. 9 shows the configuration of the FM demodulator circuit based on delay line detection that is applicable to the optical signal receiver 90. The FM demodulator circuit 92 shown in FIG. 9 comprises a limiting amplifying unit 131, a delay line 132, an AND gate 133, and a low pass filter 134.
In the FM demodulator circuit 92, the inputted frequency-modulated optical signal is shaped into a rectangular wave by the limiting amplifying unit 131. An output of the limiting amplifying unit 131 is split into two outputs; one output is inputted into an input terminal of the AND gate 133, and the other output is inputted into an input terminal of the AND gate 133 after being inverted in polarity and delayed by a time τ by the delay line 132. When this output of the AND gate 133 is smoothed by the low pass filter 134, the output will become a frequency-demodulated output (for example, see Non-patent document 2). It is known that an OR gate is also applicable instead of an AND gate (for example, see Patent document 3).
Such transmission of multichannel picture signals requires low distortion. In Non-patent document 2, CNR (Carrier-to-Noise Ratio) is set to 42 dB or more, and CSO (Composite Second-Order Distortion) and CTB (Composite Triple Beat) are set to −54 dB or less in an optical signal transmitter and an optical signal transmission system that use the FM batch conversion method.
However, in the conventional FM demodulator circuit, the delay line 132 has a characteristic that the delay line 132 has different delay times at low frequencies and at high frequencies due to impedance mismatching at both ends of the delay line 132 used for the delay line detection, or other reasons. That is, phase distortion developed between low frequencies and high frequencies. As a result, CSO and CTB will deteriorate by the phase distortion between low frequencies and high frequencies.
In the optical signal receiver using the conventional FM batch conversion method, CSO and CTB have reached to saturated values slightly exceeding −54 dB. If the FM demodulator circuit of the optical signal receiver can be configured with lower distortion, improvement in the transmission characteristic can be expected.
Patent document 1: Japanese Patent No. 2700622
Patent document 2: Japanese Patent No. 3371355
Patent document 3: Japanese Patent Application Laid-open No. 2002-141750
Non-patent document 1: ITU-T Standard J-185 “Transmission Equipment for transferring multi-channel television signals over optical access networks by FM conversion,” ITU-T
Non-patent document 2: N. Shibataetal. “Opticalvideo distribution system using FM batch conversion method,” The IEICE Transaction B (Japanese Edition), Vol. J83-B, No. 7, pp. 948-959, July 2000
Non-patent document 3: Suzuki at al. “Pulsed FM batch conversion modulation analog optical CATV distribution system,” IEICE Autumn Society Conference, Technical Digest, B-603, 1991